[Wien] Correct band gap value

Fecher, Gerhard fecher at uni-mainz.de
Thu Dec 10 12:29:15 CET 2015 

It seems your knowledge on -- just to name a few -- crystal structures, real space, Brillouin zone, symmetry, space groups, and other basics is not that high.
Those things, however, you need to know to understand what you are doing in the calculations.

F means you have a face centered cell but does not tell you much more, what do you mean why the number of symmetry opetrations changes ?

If you distort a cubic lattice (x,y,z  are equal) in one direction, say along z, you end up in a tetragonal structure and x (or y) and z are not longer the same.
That is the [001], [001] directions are not longer the same, and also [110] and [101] are not the same anymore, etc.

I realy suggest that you learn much more about crystal structures before you start to calculate something

The forum is not good for teaching textbook knowledge

Ciao
Gerhard

DEEP THOUGHT in D. Adams; Hitchhikers Guide to the Galaxy:
"I think the problem, to be quite honest with you,
is that you have never actually known what the question is."

====================================
Dr. Gerhard H. Fecher
Institut of Inorganic and Analytical Chemistry
Johannes Gutenberg - University
55099 Mainz
and
Max Planck Institute for Chemical Physics of Solids
01187 Dresden
________________________________________
Von: wien-bounces at zeus.theochem.tuwien.ac.at [wien-bounces at zeus.theochem.tuwien.ac.at] im Auftrag von Muhammad Sajjad [sajjadpu at gmail.com]
Gesendet: Donnerstag, 10. Dezember 2015 10:05
An: A Mailing list for WIEN2k users
Betreff: Re: [Wien] Correct band gap value

It looks same (in shape). I selected lattice type F and applied strain. Only symmetry operations reduce to 16 from 48 (for bulk Si). Also I kept the volume constant for every value of strain.

On Thu, Dec 10, 2015 at 11:29 AM, Fecher, Gerhard <fecher at uni-mainz.de<mailto:fecher at uni-mainz.de>> wrote:
Did you check how the Brillouin zone looks like in the tetragonal case after you applied the strain ?
Is your k-path that you used for the xmgrace plot the one that you like to have to find the correct gap ?

Ciao
Gerhard

DEEP THOUGHT in D. Adams; Hitchhikers Guide to the Galaxy:
"I think the problem, to be quite honest with you,
is that you have never actually known what the question is."

====================================
Dr. Gerhard H. Fecher
Institut of Inorganic and Analytical Chemistry
Johannes Gutenberg - University
55099 Mainz
and
Max Planck Institute for Chemical Physics of Solids
01187 Dresden
________________________________________
Von: wien-bounces at zeus.theochem.tuwien.ac.at<mailto:wien-bounces at zeus.theochem.tuwien.ac.at> [wien-bounces at zeus.theochem.tuwien.ac.at<mailto:wien-bounces at zeus.theochem.tuwien.ac.at>] im Auftrag von Muhammad Sajjad [sajjadpu at gmail.com<mailto:sajjadpu at gmail.com>]
Gesendet: Donnerstag, 10. Dezember 2015 09:19
An: A Mailing list for WIEN2k users
Betreff: Re: [Wien] Correct band gap value

Manually means I opened the band structure in xmgrace then simply clicked on the CBM and the went to the text editor to read the exact lowest value. I am simply using Silicon (with 2% strain) and the plot is along W L G X W.

On Thu, Dec 10, 2015 at 10:35 AM, <tran at theochem.tuwien.ac.at<mailto:tran at theochem.tuwien.ac.at><mailto:tran at theochem.tuwien.ac.at<mailto:tran at theochem.tuwien.ac.at>>> wrote:
What does "manually" mean? If it means band structure plotting, then
probably the VBM and/or CBM is not along the path in the Brillouin zone
that you chose.

F. Tran


On Thu, 10 Dec 2015, Muhammad Sajjad wrote:

I selected 2000 k-points. grep GAP *.scf gives 0.923 eV while manually I
found VBM at 0.0 eV and CBM at 1.10023 eV. So which one should be correct?

On Thu, Dec 10, 2015 at 12:20 AM, <tran at theochem.tuwien.ac.at<mailto:tran at theochem.tuwien.ac.at><mailto:tran at theochem.tuwien.ac.at<mailto:tran at theochem.tuwien.ac.at>>> wrote:
      If your k-mesh is not dense enough such that there is no k-point
      at the VBM and/or CBM or close to, then :GAP will be larger than
      the band gap from a band structure.

      F. Tran

      On Wed, 9 Dec 2015, Muhammad Sajjad wrote:

            Dear AllDoes the command "grep GAP *.scf" gives
            correct band gap value? as when I found the bottom
            of conduction band in xmgrace it is higher. (bottom
            of CB is
            read correctly by opening the plot in xmgrace and
            then read the date by editing in text editor).

            --
            Kind Regards
            Muhammad Sajjad
            Post Doctoral Fellow
            KAUST, KSA.


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--
Kind Regards
Muhammad Sajjad
Post Doctoral Fellow
KAUST, KSA.


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--
Kind Regards
Muhammad Sajjad
Post Doctoral Fellow
KAUST, KSA.
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--
Kind Regards
Muhammad Sajjad
Post Doctoral Fellow
KAUST, KSA.

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